Merge pull request #2308 from jpf91/aarch64

[AArch64] [GDC] AArch64: Fix backtraces in Fibers
merged-on-behalf-of: David Nadlinger <code@klickverbot.at>

Author: dlang-bot

src/core/thread.d

@@ -3678,7 +3678,11 @@ private
 
   // Look above the definition of 'class Fiber' for some information about the implementation of this routine
   version (AsmExternal)
-    extern (C) void fiber_switchContext( void** oldp, void* newp ) nothrow @nogc;
+  {
+      extern (C) void fiber_switchContext( void** oldp, void* newp ) nothrow @nogc;
+      version (AArch64)
+          extern (C) void fiber_trampoline() nothrow;
+  }
   else
     extern (C) void fiber_switchContext( void** oldp, void* newp ) nothrow @nogc
     {
@@ -4908,7 +4912,7 @@ private:
             // Only need to set return address (lr).  Everything else is fine
             // zero initialized.
             pstack -= size_t.sizeof * 11;    // skip past x19-x29
-            push(cast(size_t) &fiber_entryPoint);
+            push(cast(size_t) &fiber_trampoline); // see threadasm.S for docs
             pstack += size_t.sizeof;         // adjust sp (newp) above lr
         }
         else version (AsmARM_Posix)

src/core/threadasm.S

@@ -475,6 +475,7 @@ fiber_switchContext:
         .text
         .global CSYM(fiber_switchContext)
         .p2align  2
+        .type   fiber_switchContext, %function
 CSYM(fiber_switchContext):
         stp     d15, d14, [sp, #-20*8]!
         stp     d13, d12, [sp, #2*8]
@@ -504,4 +505,26 @@ CSYM(fiber_switchContext):
         ldp     d13, d12, [sp, #2*8]
         ldp     d15, d14, [sp], #20*8
         ret
+
+/**
+ * When generating any kind of backtrace (gdb, exception handling) for
+ * a function called in a Fiber, we need to tell the unwinder to stop
+ * at our Fiber main entry point, i.e. we need to mark the bottom of
+ * the call stack. This can be done by clearing the link register lr
+ * prior to calling fiber_entryPoint (i.e. in fiber_switchContext) or
+ * using a .cfi_undefined directive for the link register in the
+ * Fiber entry point. cfi_undefined seems to yield better results in gdb.
+ * Unfortunately we can't place it into fiber_entryPoint using inline
+ * asm, so we use this trampoline instead.
+ */
+        .text
+        .global CSYM(fiber_trampoline)
+        .p2align  2
+        .type   fiber_trampoline, %function
+CSYM(fiber_trampoline):
+        .cfi_startproc
+        .cfi_undefined x30
+        // fiber_entryPoint never returns
+        bl fiber_entryPoint
+        .cfi_endproc
 #endif